System and method for mitigating potential for formation of urea deposits in an engine exhaust system during cold ambient conditions

ABSTRACT

A device and method for catalytic reduction of NOx in gaseous products of a combustion process before entry into the atmosphere. The gaseous and particulate products of a combustion process flow radially through a radial flow particulate filter element ( 24 ) that is effective to trap particulate matter, and are then directed axially through a collector ( 26 ). An injector ( 30 ) introduces a reductant into an axial end of the collector for entrainment with axial flow through the collector in a direction away from the injector. Flow leaving the collector is directed through an SCR catalyst ( 40 ) where catalytic reduction of NOx occurs.

TECHNICAL FIELD

This disclosure relates to internal combustion engines, especially diesel engines like those used to propel large trucks, and in particular the disclosure relates to engine exhaust after-treatment that comprises injecting urea solution into the engine exhaust system for promoting selective catalytic reduction (SCR) of certain constituents of engine exhaust.

BACKGROUND OF THE DISCLOSURE

An example of a diesel engine exhaust after-treatment system that uses selective catalytic reduction (SCR) comprises an injector through which aqueous urea solution is injected into the exhaust flow. Ideally the solution should completely vaporize and thoroughly mix with the exhaust gas before the flow passes across catalytic surfaces.

The geometry of an exhaust after-treatment system and the spray pattern of a urea injector may cause some of the injected solution to wet interior surfaces of the exhaust system before it vaporizes. When the temperature of those surfaces is low enough, a potential exists for urea to come out of solution and form deposits on those surfaces. Accumulations of solid deposits may, over time, impair the effectiveness of the after-treatment system, such as by altering flow characteristics of the exhaust and/or the spray pattern of the urea injector, and/or they may damage exhaust and after-treatment system components.

Urea can potentially deposit even on the tip of the injector, and if it does so, spray quality and uniformity may be adversely affected, potentially leading to lower NOx conversion efficiency by selective catalytic reduction. Removal of significant urea deposits typically requires disassembly of components because of lack of acceptable ways to satisfactorily remove them without such disassembly.

In order to avoid wetting cold surfaces with injected solution that might cause urea to deposit on those surfaces, the injection of urea may be temporarily delayed when a cold engine is first started, especially during cold ambient conditions. That however delays the onset of exhaust gas treatment by selective catalytic reduction.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a system and method for mitigating the potential for formation of urea deposits in an engine exhaust system, especially during cold ambient conditions, e.g. below the freezing point of typical urea solution, approximately −10° C.

One or more electric heater elements are associated with certain surfaces of the exhaust system. They are effective to heat those surfaces when the potential for formation of urea deposits on those surfaces is indicated.

The surfaces include the urea injection boss, the exhaust system wall both upstream and downstream of the boss, and structure within that wall, such as a grid for breaking up spray droplets and a mixer for mixing the entrained fluids. Electric heating may occur when exhaust gas temperature, as measured by a sensor centrally located upstream of the point of injection, is lower than a selected temperature (200° C. being a typical example) below which urea injection is disallowed. The heating is continued until the measured exhaust gas temperature is high enough to reasonably assure that temperature of the surfaces being heated will be kept above some minimum above which urea deposits are unlikely to form. The power requirement of the heater element or elements is based on maintaining a minimum temperature of at least about 150° C. to about 200° C. (please explain last sentence)

The heater element or elements can be integrated with the wall of the exhaust system, specifically to heat surfaces in the direct path of the urea spray and surfaces that might have too low a temperature both prior to and during the urea injection in the absence of electric heating.

The heater can also be activated for a short time during warmer conditions, for instance if a large urea injection is performed in order to more quickly charge the SCR catalyst.

The disclosed system and method can mitigate the potential for urea deposit formation and/or decrease the amount of time for which urea injection is disallowed when a cold engine is first started.

A general aspect of the disclosure relates to an internal combustion engine comprising an exhaust system through which exhaust gas created by combustion in engine combustion chambers passes to atmosphere and which comprises an after-treatment device for treating the exhaust gas before the exhaust gas leaves the exhaust system, an injector for injecting a liquid phase reductant for the after-treatment device's treatment of the exhaust gas into the exhaust system upstream of the after-treatment device for entrainment with, and vaporization in, flow of the exhaust gas through the exhaust system, and one or more electric heater elements for heating a surface that may be wetted by the liquid phase to a temperature that is high enough to prevent reductant from coming out of the liquid phase and depositing on the surface.

Still another general aspect relates to a method for mitigating the deposit of reductant out of liquid phase on a surface of an internal combustion engine exhaust system through which exhaust gas created by combustion in engine combustion chambers passes to atmosphere and which comprises an after-treatment device for treating the exhaust gas before the exhaust gas leaves the exhaust system, and an injector for injecting liquid phase reductant for the after-treatment device's treatment of the exhaust gas into the exhaust system upstream of the after-treatment device for entrainment with, and vaporization in, flow of the exhaust gas through the exhaust system.

The method comprises operating one or more electric heater elements to heat a surface that may be wetted by the liquid phase to a temperature that is high enough to prevent reductant from coming out of the liquid phase and depositing on the surface.

The foregoing summary, accompanied by further detail of the disclosure, will be presented in the Detailed Description below with reference to the following drawings that are part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic diagram of an engine and its exhaust system, including after-treatment.

FIG. 2 is a schematic diagram of a urea spray pattern in the exhaust system.

DETAILED DESCRIPTION

FIG. 1 shows an example of a turbocharged diesel engine 10 having an intake system 12 through which charge air enters and an exhaust system 14 through which exhaust gas resulting from combustion exits, not all details of those two systems that are typically present being shown. Engine 10 comprises a number of cylinders 16 forming combustion chambers into which fuel is injected by fuel injectors to combust with the charge air that has entered through intake system 12. Energy released by combustion powers the engine via pistons connected to a crankshaft.

When used in a motor vehicle, such as a truck, engine 10 is coupled through a drivetrain to driven wheels that propel the vehicle. Intake valves control the admission of charge air into cylinders 16, and exhaust valves control the outflow of exhaust gas through exhaust system 14 and ultimately to atmosphere. Before entering the atmosphere however, the exhaust gas is treated by one or more after-treatment devices in an after-treatment system 18.

After-treatment system 18 comprises a walled housing 20 circumscribing an exhaust flow path and having an exhaust gas inlet 22 through which exhaust gas from cylinder 16 enters for passage through the housing. The interior of housing contains, in succession along the exhaust flow path from inlet 22, a diesel particulate filter (DPF) element 24, a perforate impingement plate, or mesh 26, and a mixer 28.

A urea injector 30 is mounted in a boss 32 in the housing wall for spraying aqueous urea solution as a liquid phase reductant from a nozzle 34 into the exhaust gas flowing through the housing. Flow that has passed through mixer 28 is conveyed to pass across catalytic surfaces of an SCR catalyst 36 where catalytic reduction of NOx by the reductant occurs before the flow exits the exhaust system through a tailpipe.

A supply of aqueous urea solution is stored in a tank 38. A representative urea solution has approximately a 32.5% concentration by weight.

The injection of solution into the exhaust flow is controlled by a controller 40 that is associated with the supply in tank 38 and with injector 30. A control input to controller 40 is a measurement of exhaust gas temperature obtained from a temperature sensor 42 that is disposed to measure temperature of exhaust gas upstream of the injector at a generally central location of the flow.

Injector nozzle 34 lies substantially on an imaginary centerline aimed downstream of the flow, but at an acute angle to the prevailing axial flow coming from filter element 24. Injector 30 injects urea solution as a spray having a representative pattern 44 shown in FIG. 2.

Plate, or mesh, 26 is disposed in the spray path and intended to promote rapid vaporization of droplets in the urea solution spray by deflecting/dispersing/breaking up the droplets. Mixer 28 is intended to promote thorough mixing of the vapor with the exhaust gas flow before it arrives at SCR 36. SCR catalyst 40 promotes the reaction of exhaust gas constituents with the decomposition products of urea solution vaporization.

One or more electric heater elements, schematically indicated by the dot-dash line 46 in FIG. 1 are associated with certain surfaces of the exhaust system. The heater elements are intended to heat those surfaces when the potential for formation of urea deposits on those surfaces is indicated. The surfaces include boss 32, the wall of housing 20 both upstream and downstream of boss 32, plate, or mesh 26, and mixer 28.

A representative use of heater elements 46 occurs when exhaust gas temperature, as measured by sensor 42, is lower than a selected temperature below which controller 40 disallows urea injection because of concern that deposits could form on surfaces wetted by the liquid phase reductant. The heater elements are activated, with heating being continued until the measured exhaust gas temperature is high enough to reasonably assure that temperature of the surfaces being heated will be kept above some minimum above which urea deposits are unlikely to form.

The heater elements can be integrated with the housing wall and boss, specifically to heat surfaces in the direct path of the urea spray and surfaces that might have too low a temperature both prior to and during the urea injection in the absence of electric heating. For example a heater element in the boss can have a high enough rating to cause heat to flow through the boss and into the injector through which liquid passes.

The heater can also be activated for a short time during warmer conditions, for instance if a large urea injection is performed in order to more quickly charge the SCR catalyst.

The disclosed system and method can mitigate the potential for urea deposit formation and/or decrease the amount of time for which urea injection is disallowed when a cold engine is first started. 

1. An internal combustion engine comprising: an exhaust system through which exhaust gas created by combustion in engine combustion chambers passes to atmosphere and which comprises an after-treatment device for treating the exhaust gas before the exhaust gas leaves the exhaust system; an injector for injecting a liquid phase reductant for the after-treatment device's treatment of the exhaust gas into the exhaust system upstream of the after-treatment device for entrainment with, and vaporization in, flow of the exhaust gas through the exhaust system; and one or more electric heater elements for heating a surface that may be wetted by the liquid phase to a temperature that is high enough to prevent reductant from coming out of the liquid phase and depositing on the surface.
 2. An engine as set forth in claim 1 in which the after-treatment device comprises an SCR catalyst, and further including a supply of urea solution for the injector to inject as the liquid phase.
 3. An engine as set forth in claim 2 in which the injector comprises a body that is received in a bore of a boss and a nozzle through which urea solution is sprayed into the exhaust gas flow, and in which a heater element is disposed on the boss for heating the boss.
 4. An engine as set forth in claim 2 in which a heater element is disposed on a portion of a wall of the exhaust system that circumscribes the exhaust gas flow and is downstream of the boss.
 5. An engine as set forth in claim 2 in which a heater element is disposed on a portion of a wall of the exhaust system that circumscribes the exhaust gas flow and is upstream of the boss.
 6. An engine as set forth in claim 2 in which the exhaust system comprises structure disposed in the path of urea spray from the nozzle for deflecting/dispersing/breaking up at least some portion of the spray before that portion of the spray can contact the surface, and in which a heater element is disposed on a wall of the exhaust system that circumscribes the exhaust gas flow at the location of the structure.
 7. An engine as set forth in claim 6 in which the exhaust system further comprises a mixer downstream of the structure, and a heater element is disposed on the wall of the exhaust system at the location of the mixer.
 8. An engine as set forth in claim 2 in which a temperature sensor is disposed to measure temperature of exhaust gas upstream of the injector, and in which a controller for the one or more heater elements uses that measurement to control operation of the one or more heater elements.
 9. A method for mitigating the deposit of reductant out of liquid phase on a surface of an internal combustion engine exhaust system through which exhaust gas created by combustion in engine combustion chambers passes to atmosphere and which comprises an after-treatment device for treating the exhaust gas before the exhaust gas leaves the exhaust system, and an injector for injecting liquid phase reductant for the after-treatment device's treatment of the exhaust gas into the exhaust system upstream of the after-treatment device for entrainment with, and vaporization in, flow of the exhaust gas through the exhaust system, the method comprising: operating one or more electric heater elements to heat a surface that may be wetted by the liquid phase to a temperature that is high enough to prevent reductant from coming out of the liquid phase and depositing on the surface.
 10. A method as set forth in claim 9 in which the step of operating one or more electric heater elements comprises operating an electric heater element disposed on a boss having a bore within which a body of the injector is received.
 11. A method as set forth in claim 9 in which the step of operating one or more electric heater elements comprises operating an electric heater element disposed on a portion of a wall of the exhaust system that circumscribes the exhaust gas flow and is downstream of the boss.
 12. A method as set forth in claim 9 in which the step of operating one or more electric heater elements comprises operating an electric heater element disposed on a portion of a wall of the exhaust system that circumscribes the exhaust gas flow and is upstream of the boss.
 13. A method as set forth in claim 9 in which the step of operating one or more electric heater elements comprises operating an electric heater element disposed on a portion of a wall of the exhaust system that circumscribes the exhaust gas flow at the location of structure disposed in the path of urea spray from the nozzle for deflecting/dispersing/breaking up at least some portion of the spray before that portion of the spray can contact the surface.
 14. A method as set forth in claim 13 in which the step of operating one or more electric heater elements comprises operating an electric heater element disposed on a portion of a wall of the exhaust system that circumscribes the exhaust gas flow at the location of a mixer downstream of the structure.
 15. A method as set forth in claim 9 further comprising measuring temperature of exhaust gas upstream of the injector, using the measured temperature to selectively allow and disallow the injector to inject urea solution and to selectively operate the one or more heater elements. 